US 7839232 B2
A broadband transmission line impedance transformer performs impedance transformation with improved frequency response and efficiency across a wide operational bandwidth. In particular, the bandwidth of a transmission line 2:1 impedance transformer may be significantly increased by adding an additional compensating capacitor as an internal component between interconnected transmission lines. This capacitor effectively improves low frequency response for a given length of transmission lines and decreases mismatch in an entire frequency range. The overall bandwidth ratio increases at least twice and mismatch decreases.
1. A transformer having enhanced frequency response, comprising:
a balun transformer comprised of a pair of interconnected four conductor transmission lines, said balun transformer having an input port and a balanced output port; and
two compensating capacitors coupled inside the balanced part of the transformer;
wherein the compensating capacitor improves low frequency response and decreases mismatch in an entire frequency range.
2. The transformer according to
3. The transformer according to
This application is a divisional of Ser. No. 11/224,972 filed Sep. 14, 2005 now U.S. Pat. No. 7,583,160, which claims priority to U.S. Provisional Application No. 60/610,692 filed on Sep. 17, 2004 entitled “Broadband Transmission Line Transformer” by Simon Y. London.
1. Field of the Invention
This invention relates generally to broadband radio-frequency impedance transformers. More particularly, the invention relates to broadband transmission line transformers with non-integer turns ratio (fractional ratio transformers) and mostly for high power application.
2. General Description of the Prior Art
A particular class of RF impedance transformers with maximum achievable bandwidth and low insertion losses is a class of transmission line transformers that plays an important role in various RF systems, from low power up to high power levels.
The main frequency limitation factors of these transformers are shunt inductance at lower frequencies and electrical length of transmission lines at higher frequencies. These two contradictory factors determine the achievable bandwidth of transformers. Impedance transformers with diverse circuit models, having different interconnections of transmission lines and impedance transformation ratios, have different limitations influenced by these two factors. As result, greater or lower bandwidth can be achieved.
Widely used impedance transformation ratios are 4:1, 9:1 and 16:1 (integer turns ratios), and 1.5:1, 2:1, 3:1 (fractional turn's ratios). The latter ones are more difficult to realize at wide bandwidths, especially for high power application.
Various circuit diagrams of transmission line impedance transformers are presented in book of Jerry Sevick “Transmission Line Transformers.” Among the impedance transformers with non-integer turns ratios, the most necessary is 2:1 impedance transformer. A typical structure includes a two-way power combiner/divider, which consists of a combiner/divider itself and a 2:1 impedance transformer.
All of these RF transformers have multi-octave bandwidth and use generally ferrite toroids or other ferrite configurations. Due to high magnetic permeability of ferrite transformers, shunt inductance is high enough and it is possible to realize multi-octave bandwidth with admissible electrical length of transmission lines.
In high power transformers (5-100 kW), magnetic flux in ferrite is also high and introduces harmonics and intermodulation products.
Furthermore, for these transformers, hysteresis losses (heat dissipation) limiting power handling capability may require a liquid cooling system. Such transformers are heavy, expensive and can not be used in certain environmental conditions.
Many attempts to develop high power, broadband transformers without ferrite have been made. In this case the high-pass correction usually used for partly compensation of relatively small shunt inductance. In simplest case it may be one series connected capacitor at the input or at the output of transformer.
In spite of this, for achieving multi-octave bandwidth, especially at high power, the electrical length of the transformer's transmission lines should be great and high frequency limitation occurs. Additional low-pass correction compensates this effect to some extent. All of these corrections make transformers more complicated and expensive.
In addition, for transformers with fractional turn ratios, the impedance ratios in some practical cases are not close enough to integer numbers and, consequently, even if the transformer is ideal some mismatch occurs. For example, when typical turns ratio is 3/2, the corresponding impedance's ratio is 2.25, and with respect to required impedance transformation ratio equal to the calculated VSWR=1.125. Practically, in this case value of VSWR will be higher.
Furthermore, in combining the power of several amplifiers, a two-or more stage combining system is usually used. If each stage inserts some particular VSWR, the overall VSWR in the worst case is a product of its individual values. To decrease the above-mentioned theoretical value, the turns ratio 7/5 instead of 3/2 may be used, for example. A corresponding transformer is too complicated, especially for high power application. Besides, admissible electrical length of its transmission lines should be relatively small and the highest operating frequency decreases.
The factors discussed above are applicable to impedance transformers that are unbalanced, balanced and baluns (balanced-to-unbalanced). Frequently it is difficult to provide good balance for high power, broadband baluns, especially for fractional turns ratios.
In a prior art balun with 2.25:1 impedance ratio (U.S. Pat. No. 5,767,754), an additional transformer winding is used to improve balance. This winding introduces capacitive shunt effect that increases mismatch. Besides, balance can not be perfect due principally to the asymmetry of circuit models and the influence of stray elements, especially for high power applications. Different longitudinal voltages on the windings also introduce additional difficulties at high power levels.
Another approach is a chain connection of two transformers in different combinations (see books of Jerry Sevick: “Transmission Line Transformers” and “Building and Using Baluns and Ununs,” CQ Communications Inc., 1994). This approach is too complicate at high power levels and the balance is not good enough due to stray elements in real design.
In view of the above, it is an object of the present invention to provide a more effective, high power broadband impedance transformer.
It is another object of the present invention to provide a high frequency, high power transformer with unbalanced ports that is simple in construction and has a wide bandwidth without ferrite.
It is still a further object of the present invention to provide an unbalanced impedance transformer without ferrite having a multi-octave bandwidth ratio up to 20:1.
Still another object of the present invention is to provide a high power, broadband unbalanced transformer with a fractional turns ratio, and specifically to provide a 2:1 impedance transformation ratio.
Yet another object of the present invention is to provide a broadband, unbalanced transformer with a simple correction.
It is still a future object of the present invention to provide a broadband, unbalanced impedance transformer having very small mismatch with respect to standard nominal port impedances.
It is another object of the present invention to provide a high frequency, high power transformer with balanced ports that is simple in construction and has wide bandwidth without ferrite.
It is still a future object of the present invention to provide a broadband balanced-to-unbalanced impedance transformer (balun) having all above mentioned properties and good balance in entire frequency band.
According to the present invention, a significant increase in bandwidth and a simplifying, multi-octave impedance transformer are achieved. These results are obtained by combining two factors in one device:
This capacitor, together with shunt inductance of transmission lines, effectively decreases mismatch in the entire frequency band caused by 3/2 turn's ratio.
The described effect takes place for unbalanced-to-unbalanced transformers, for balanced-to-balanced transformers and for balanced-to-unbalanced transformers (baluns).
The above described features and advantages of the present invention will be more fully appreciated with reference to the detailed description and figures, in which:
At high power and in a broadband application, where efficiency is an important factor, transmission line impedance transformers are the best in most cases of HF-VHF frequency bands. These transformers generally have a simple construction.
The nominal impedance at port 11 with respect to common ground 13 is 2.25 times more than the nominal impedance at port 20 with respect to ground 13. Consequently, this unbalanced transformer with fractional 3/2 turns ratio, even if ideal, implies VSWR=1.125. Shunt inductance increases this value at lower frequencies.
Besides, this transformer can operate satisfactorily if electrical length each of its transmission line does not exceed 60 deg at upper operating frequency. Corresponding optimum characteristic impedances of two coax cables 14 and 17 are different and non-standard values. For equal or standard values of characteristic impedances maximum admissible electrical length decreased rapidly.
Another electrical scheme of simple impedance transformer with the same impedance transformation ratio 2.25 and near the same achievable frequency characteristics is shown on
Ideally, this transformer with unbalanced ports 31 and 32 with respect to common ground 36 is operable at an unlimited upper frequency. On the other hand, it consists of two separate shunt inductances, formed by outer conductors of lines 33 and 34, and of three separate transmission lines. Implementation of this transformer in high power applications introduces stray inductances and capacitances that decrease the upper operating frequency.
Moreover, at some electrical length, all transmission lines have a resonance cut-off frequency that may occur. As a result, these transformers are relatively complicated and operate also at limited electrical length of transmission lines.
Another prior art transformer (
This 2.25:1 ratio impedance transformer with two unbalanced ports 51 and 52 with respect to common ground 53 has the same characteristic impedance of both lines 54 and 57. The line 54 with inner conductor 55 and outer conductor 56 corresponds to line 32 on
All transformers shown on
With this three-element correction, the transformers in U.S. Pat. No. 5,309,120 provide bandwidth ratio up to 5:1. They can operate satisfactorily at electrical length of lines significant less than 90 deg.
Referring now to
Effectively compensates shunt inductance of paired outer conductors 86 and 89, and
Decreases inserted mismatch due to 3/2 turns ratio in a wide frequency band.
The optimum characteristic impedance of each of the coax cables 84 and 87 is equal Z0✓2, where Z0 is nominal impedance at port 82 (lower impedance side).
For transformers with a typical required 50:25 Ohm impedance transformation, the characteristic impedance of each coax, Z=35.35 Ohm, i.e., is practically 35 Ohm. Manufactured coax cable UT 141-35 has Z=35 Ohm.
Capacitor 83 in this transformer is connected between the end of inner conductor 85 of the first line 84 and port 82. On the other hand, this capacitor is connected inside the transformer and between the first turn 85 and the second turn 88. The third turn is formed by connecting together outer conductors 86 and 89 of coax cables 84 and 87.
Capacitor 83, together with the inductance of paired outer conductors 86 and 89, forms a high-pass filter that also improves frequency response. As a result, this transformer has the following advantages:
Simple in construction (includes paired coax that have equal characteristic impedances),
Includes only one correcting element,
Operates satisfactorily up to electrical length of each coax 110 deg, and
Provides low reflection by relatively low shunt inductance.
The calculated value of reflection coefficient is ISImax 0.035 in cases of a 2:1 impedance transformation ratio.
Nominal impedances at ports 94 and 95 with respect to common ground 96 are also the same as for
The optimum characteristic impedance of each coax cable 103 and 105 is equal to Z/42, where Z is the nominal impedance at balanced port 102-102′ (lower impedance side). For a transformer with 100:50 Ohm impedance, the transformation characteristic impedance of each coax is equal Z=35.35 Ohm, i.e., practically 35 Ohm.
Two capacitors 107 and 108 have identical values of capacitances. They compensate shunt inductance of two pairs of outer conductors of coax cables 103-104 and 105-106. The calculated reflection coefficient with these capacitors and with relatively small shunt inductance is 0.03 in the case of a 2:1 impedance transformation ratio.
Now referring to
Now referring to
Experimental 20:1 Bandwidth Ratio Transformer
The laboratory prototype of an 50:25 Ohm impedance transformer was constructed without ferrite in accordance to
A two-way power combiner consists of two cables FE 81 connected in parallel at common port 4 (
The calculated upper operating frequency is equal z-, 43.5 MHz, i.e. enough close to an experimental result for a full device (transformer with combiner itself). Data on
While the devices and methods of this invention have been described in terms of specific embodiments, it will be apparent to those of skill in the art that variations may be applied to the devices without departing from the concept, spirit, and scope of the invention. Therefore, all such substitutions and modifications apparent to those skilled in the art are deemed to be within the spirit, scope, and concept of the invention as defined by the appended claims.